1. Field of the Invention
The present invention relates to a laser irradiating apparatus which performs annealing using a laser beam (hereinafter called xe2x80x9claser annealingxe2x80x9d) and which includes a laser and an optical system for guiding a laser beam emitted from the laser to an object to be processed.
2. Description of the Related Art
A glass substrate is advantageous over a synthetic quartz glass substrate, which has been popular conventionally, in inexpensiveness and high processability. It is possible to easily produce a large substrate of, for example, 600 mmxc3x97720 mm from a glass substrate. This is one reason why studies on a glass substrate are being made. The reason for the use of a laser in annealing is that annealing can be carried out without significantly raising the temperature of the substrate so that a glass substrate with a low melting point can be used. In addition, laser annealing has a considerably a high throughput as compared with heating means using an electric heating furnace.
A method which executes laser annealing by using an optical system to process a pulsed laser beam from a high-power laser, such as an excimer laser, into a rectangular spot of several centimeters on each side or a linear shape of 10 cm or longer in length on an irradiation surface and scanning a target with the laser beam (or moving the irradiation position of the laser beam in relative to the irradiation surface) is used on purpose because of its high productivity and industrial advantage. The xe2x80x9clinear shapexe2x80x9d mentioned here does not mean a xe2x80x9clinexe2x80x9d in a strict sense, but means a rectangle having a large aspect ratio (or an elongated ellipse). For example, the xe2x80x9clinear shapexe2x80x9d indicates an elongated shape of an aspect ratio of over 10 (preferably 100 to 10,000) and a beam with such a shape is called a xe2x80x9clinear beamxe2x80x9d.
When a linear beam is used, particularly, unlike in the case of using a laser beam spot that needs scanning in the forward, backward, right and left directions, the entire irradiation surface can be irradiated with the beam in scanning only in a direction perpendicular to the major axis of the linear beam. This leads to a high productivity. Scanning is carried out in the direction perpendicular to the major axis because it is the most efficient scanning direction. Because of the high productivity, the use of a linear beam acquired by processing a pulse-oscillated excimer laser beam by an adequate optical system in laser annealing is becoming a mainstream in the manufacturing technology at present.
For example, laser annealing can be executed by using a YAG laser. The YAG laser uses a non-linear optical element to modulate a laser beam into the second harmonic and further uses an optical system to process the second harmonic into a linear beam. The energy distribution of the linear beam is made to be a Gaussian distribution in the direction of the minor axis and be uniform in the direction of the major axis (see FIG. 2). But, the YAG laser produces a coherent beam which has an extremely high coherence. The coherent length of the YAG laser is about 10 mm whereas the coherent length of the excimer laser is several micrometers to several tens of micrometers. It is therefore difficult to form a YAG laser beam having a uniform energy distribution on or near the irradiation surface.
Accordingly, the invention aims at providing a laser irradiating apparatus which forms a laser beam having an improved and uniform energy distribution on or near its irradiation surface.
Laser beams have such a characteristic that the beams, even if emitted from the same light source, do not interfere with one another as long as there is an optical path difference equal to or greater than the coherent length. A YAG laser has a very high coherence as compared with an excimer laser. The coherent length of the YAG laser is around 10 mm whereas the coherent length of the excimer laser is several micrometers to several tens of micrometers.
The invention is characterized in that a laser beam having a cyclic energy distribution is formed by intentionally forming interference fringes on or near the irradiation surface by utilizing the high coherence of a YAG laser, thereby ensuring cyclic repetition of the energy distribution of the laser beam. From a macro viewpoint, such a laser beam can be considered as having a uniform energy distribution.